The development of increased pulmonary resistace during prolonged bypass pumping is a problem of great importance both clinically and in research concerned with developing artificial circulatory assist devices. From available data, it appears reasonable that leukocyte damage by shear stress or foreign material surface exposure is at least a principal cause of this problem. This research will quantitate the changes in leukocyte function, mechanical properties and morphology caused by in vitro exposure to shear stress and foreign surfaces under precisely controlled viscometric conditions. With these damage levels ascertained, in vivo measurements of the hemodynamic change and morphological change caused by infusion of leukocyte suspensions will be taken. Cells which have undergone a known amount of in vitro trauma will be injected into experimental animals. The relationship obtained between level and type of in vitro leukocyte damage and the rate of increase in in vivo pulmonary resistance will allow the development of design criteria for circulatory assist devices.